It is the long-range purpose of this work to clarify the mechanism of cell transformation by studying the genetic and biological factors affecting carcinogenesis by chemicals. This work also aims to clarify the role of these factors in the etiology of human cancer incidence. The topics of present interest are: 1) the frequency of chemical transformation of the cells derived from xeroderma pigmentosum patients who are predisposed to a higher incidence of cancer; and 2) the characterization of cell variants which shows the different sensitivity to chemical transformation.